Any reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge in Australia or elsewhere.
The expectation of consumers is that supermarkets and grocery outlets will provide a regular seasonal supply of fruits such as bananas, apples, mangoes and avocados in or near to a ripened state. This means that the fruit must be in a state, in terms of firmness, sweetness and color, close to being ready for consumption and hence desirable to the consumer.
The challenge in achieving this is that, often, the fruit will be grown in a region which is geographically distant from the point of sale. In countries such as Australia, fruit may be transported over extremely long distances by road and, in the case of imported fruits, long transport times by boat are often necessitated. Fruit which was already in a ripened state when transported would be over ripe by the time it reaches its destination and, due to the softness of ripe fruit, a significant percentage of the fruit would be damaged in transit.
For these reasons many climacteric fruits are harvested in an unripe state. The fruit will therefore be relatively hard and can be transported safely. The fruit will then be delivered to dedicated ripening rooms to encourage ripening prior to distribution to retailers.
Ripening rooms create a controlled environment that attempts to replicate the climatic conditions of heat and humidity where the fruits were grown. The ripening effect is brought about in the main by the introduction of ethylene gas into the ripening rooms. Ethylene is released naturally by many fruits, such as bananas for example, and is a known ripening agent.
The use of ripening rooms does present a number of disadvantages. Firstly, it adds significantly to the length of the supply chain resulting in lost sales and profits to fruit producers and retailers, due to the delay, and often poor quality fruit. This is particularly accentuated where there is limited supply at the start of the season when prices are their highest. Secondly, the operators of the ripening rooms charge 20-30 cents per kg to ripen fruit which reduces the efficiency, profitability and transparency of the supply chain and consolidates control to a few market agents. Thirdly, not all growers are able to have consistent access to ripening rooms due to a small presence in the market or the cost being prohibitive and so growers of, for example, stone fruits, may end up providing unripened fruit to the market. This puts consumers off further buying of that fruit and can significantly damage the market for it. Finally, those growers in developing countries may not have the option of sophisticated ripening rooms at all due to the lack of infrastructure.
It would be useful to be able to ripen the fruit in an alternative manner such that it can be delivered direct to the retailer without the delay involved in the use of ripening rooms. This would allow the fruit to be transported in a relatively hard unripe state and soften only at the end of transportation thereby minimising damage.
The problem is that high levels of ethylene in the trucks would be dangerous due to its high flammability. Further, modification of the trucks may be necessary to contain the ethylene levels as it would be dispensed very quickly from the cylinder. There are also inherent dangers in carriage of pressurised cylinders.
Molecular encapsulation techniques have been used to trap ethylene as a guest molecule inside a macromolecular host, such as cyclodextrin (CD). However, current methods require very high pressures over relatively long time periods to achieve fairly modest loading of the ethylene into the CD. Ho et al (Food Chemistry 127 (2011) 572-580) provided a means for incorporating ethylene into α-CD to form a crystalline inclusion complex showing an approximate 1:1 molar ratio of ethylene to CD. The method employed involved the dissolution of α-CD in water followed by exposure to ethylene gas in a pressurised environment. It was found that the yield of the crystalline ethylene/α-CD inclusion complex increased, to a point, with increasing pressures and time. However, the maximum yield obtained was less than 45% after 5 days at a pressure of 1.5 MPa.
It would be useful to provide a controlled release ethylene source which could be safely used to ripen fruit in transit and which could be obtained at relatively low cost.